Scandoubler/flickerfixer

[Lemmink]

I have heard that you can get an external graphics card for the A1200D. Is that correct ?

Yes, that will be the graffiti http://www.amiga-hardware.com/graffiti.html

Unfortunately it never get really supported. There are only three games (Trapped I / II, Nemac IV), Shapeshifter and some imageviewers that make use of it.

On the subject of the scandoubler I would advise you get an external one, that can be used with every Amiga in case you ever switch machines. I made good experience with the external Scandex  http://www.amiga-hardware.com/scandex.html

[Lemmink]

But an internal one doesn't have better quality?

Sure has, as the signalways are much shorter (how much closer can you get pulling them right from the chips) but it will require to open the case and he didn`t sound that familar with the guts of the A1200.
From my experience the external ones don`t lack that much signalquality, plus you can "recycle" it when useing an other Amiga.